"John Keeney" wrote in message
...

"Tony Williams" wrote in message
m...
The ideal gun for aerial combat will of course combine the best of all
worlds: a high rate of fire, instantly achieved; a high muzzle
velocity to minimise flight time; and projectiles large enough to
inflict serious damage with each hit (requiring a calibre in the 25 -
30 mm range). The optimum weapon among those currently developed may
well be the new GIAT 30M791 revolver, although its weight means that
two GSh 301s (or a GSh-30) could be carried instead, with a higher
rate of fire. If the Russian guns' 30 x 165 ammunition were loaded
with lighter projectiles for a higher muzzle velocity, its aerial
combat capabilities would be improved, at the cost of some loss of
ground attack effectiveness."

Is there more behind the "requiring a calibre in the 25-30 mm range"
than hand-waving to dismiss 20mm guns? Granted, bigger is better,
but why isn't 40mm required or 20mm enough?

One reason is range but that's a mug's game, trying to compete with SRAAMs.
The other reason, lethality, is driven by the fact that fighters are a lot
tougher targets now than they were fifty years ago. In a progression of
lethality, during WWII, .50cal machine guns were adequately lethal against
fighter sized targets but not against bombers. With the advent of jet
propulsion, increased air speed required stronger structure and fighters got
physically tougher, so 20mm was optimum against fighters in the late forties
and early fifties. Supersonic fighters are tougher still, mostly because of
increase design dynamic pressure but also because they are stressed for
larger loads and higher g-loads, so the thought is that 20mm rounds have
inadequate Pk (given a hit).

Of course, gun installations are questionable now because the SRAAMs have
gotten so good and because guns in general pose a significant cost in
reliability (the firing forces become the design environment for all the
electronics in the vicinity) and a significant maintenance burden.